This invention relates generally to PLLs (phase locked loops) and specifically to PLLs that are used in FPLLs (frequency and phase locked loops) that have an I (in-phase) channel, including a demodulated pilot, and a Q (quadrature) channel.
In digital VSB (vestigial side band) television signals, a small pilot is transmitted to aid in initially receiving the television signal and subsequently locking to the received television signal. When the television signal is demodulated, the pilot is in the form of a DC offset. The VSB receiver optimally has an FPLL, that includes a PLL. The PLL has a bandwidth that is dependent upon pilot level and is therefore sensitive to noise and other channel impairments (such as a xe2x80x9cghostedxe2x80x9d 180xc2x0 out of phase pilot) which reduce the pilot level and cause a narrowing of the PLL bandwidth. The narrowed bandwidth results in reduced tracking-out-of burst and phase noise components that are associated with the pilot. Such phase and burst noise components can arise from tuners and other heterodyning devices that have noisy oscillators.
In a digital VSB receiver, excess burst and phase noise must be removed by the digital phase tracker prior to data slicing. Optimum performance depends upon achieving a balance between the operation of the pilot-tracking PLL and the digital phase tracker. Consequently, stabilization of the bandwidth of the pilot-tracking PLL is highly desired. The present invention provides a stabilization system for the PLL.
A principal object of the invention is to provide a novel PLL bandwidth stabilization system.
Another object of the invention is to provide a VSB television receiver having an FPLL with a bandwidth stabilized PLL.
A further object of the invention is to provide an improved VSB television receiver.